High frequency signaling



Eat; 8, 1931 B. w. KENDALL 1,835,783

HIGH FREQUENCY S IGNALING Original Filed Nov. 9, 1916 5 Sheets-Sheet l //\/VENTOP B. W KENDALL ATTORNEY Dec. 8, 1931. 3 w, KENDALL 1,835,783

HIGH FREQUENCY SIGNALING Original Filed Nov. 9, 1916 5 Sheets-Sheet 2 T l m IHHHHH wms/vrop B. Wife/man.

Dec. 8, 1931. B. w. KENDALL 1,835,783

HIGH FREQUENCY SIGNALING Original Filed Nov. 9, 1916 5 Sheets-Sheet Z5 B. W KENDALL Dec. 8, 1931. B. w. KENDALL HIGH FREQUENCY SIGNALING Original Filed Nov. 9, 1916 5 Sheets-Sheet 4 lNVEA/TOP 5. WKENQA fiece W31. a. w. KENDALL HIGH FREQUENCY SIGNALING Original Filed Nov. 9, 1916 5 Sheets-Sheet 5 A TTO/P/VEY Patented Dec. 8, 1931 nnrrzen stares narsnrorrlcs enson KENDALL, or nasr onanennnw JERSEY, ASSIGNOR T0 WESTERN EL G- T310 ooivrriinr, moonronarnn, or ennw ronn, my, A oonr'onA'rIon OF NEW YDRK HIGH FREQUENCY SIGNALING original application filed November 9, 1916, Serial No. 130,850. 'latent no. 1,773,901, dated August 26, 1930.

Divided and this application filed October 26, 1928. Serial No. 315,121. I

Th s division of my application Serial ,lko. 130,350, filed November 9, 1916, patented August 26, 1030, No. 1,773,901, for-high frequency signaling.

l'his invention relates to high frequency signal in which signals are transmitted as modi ations ofa high frequency wave.

.11 United States Patent- 1,330, l71 to B. \V. Kendall, patentel February 10, 1920, for high rcqnency signaling, it is shown how the li-zni'iodyne method of reception is adapted to telephonic signaling. According to this method. the detected signal is intensified by comb the received modulated oscillat'. h locally generated oscillations of a frequency the same as the carrier frequency. The oscillati -is generated at the receiving st tion will hereafter be termed reinforcing us for brevity and to differentiate em from the carrier oscillations generated at e sending station. If the carrier oscilns have a frequencywhich is notequal in va ue to the reinforcing frequency, beats are produced having a frequency equal to the difference in the frequencies of thecarricr and lfe reinforcing oscillations. These l: festcd a sound or noise which a e n rpernnposed upo-nthe signal being ire "ed, and this interferes with a clear and distinct reception of the signal.

To prevent a masking of the signal by such a sound or noise, the present invention has for an object to provide method and means for insuring that the carrier and reinforcing frequencies shall have identical values.

Th is is accomplished by generating control oscillations, and by changing the frequency thereof to providea source of carrier oscillation s whi ch :1 re modulated in accordanceiwith a signal. to be transmitted. The control oslions and the modulated oscillations are it to the receiving station wherethe frequat-nc of the control oscillations is changed, in the same manner as it was at the sending station, to provide a source of reinforcing oscillations which are combined with the received modulated oscillations to reproduce the signal. control oscillations from which both the car'- rier and reinforcing oscillations are derived,

Since there'is a single source or.

the carrier and reinforcing oscillations will always have thesain'o frequency which may,

lmwever, assume different values,,due to vari- The frequency changer used in, the systems above described is a harmonic generator which givesonly a relativclysmall number of frequencies within a given frequency range, since there are generated only those frequencies which are multiples of the frequency supplied to the harmonic generator. 1 Uthcr objects of the invention are to make 0. lai r nnmber of freq ucncies available withill a on frequency range than wouldbe possi ole 'i only a single control frequency were used, and to provide frequencies which are not multiples each other. a

g This is accomplished by generating two fre quencies, for instance, oscillations of 3000 and 4000 cycles, which are. individually supplied to harmonic generators,each of which gives any desired multipleof the frequency supplied to it. The third multiple, 9000,,for example, from the 3000:cycle harmonic generator may be combined in a thermionic d6? vice with the fourth .multiple, 16,000, supplied by the i000 cycle harmonic generator to produce a frequency equal to their sum, 9000 plus 16,000, or 25,000, which is a frequency different from that'of any multiple supplied by either of the harmonic generators.

This will be further considered in connection with the'drawings, in whichFigs. 1 and 2 show a wirelesssending and receiving station respectively, in which control oscillations supplied at the sending station are emtions and the reinforcingoscillations. Figs.

3 and 4: show diagrammatically a sending and receiving station respectively of a multiplex system, in which the carrier and reinforcing frequencies used are multiples of the control frequency. F 5 and show a sending and receiving station respectively of a multiples system, in which the number of carrier frequencies is increased by combining the multiple frequencies from two harmonic generators, and frequencies are obtained which are not multiples of each other.

f Referring to Fig. 1, the oscillationgcne rator O, of the general type disclosed and claimed in the patent to Hartley, oscillation generators, 1,356,763 patented October 26, 1920, suppliescontrol oscillations and comprises a three-element vacuum tube of the audion type, in which the input circuit is inductively connected to the output circuit by means of the coils 2 and The space current in this tube is supplied by the battery 4. By means of the variable condenser 5 shunted across the coils 2 and 3, the fre quency supplied by the 'enerator may be given any desired value. This oscillation generator which supplies control oscillations is connected to the input circuit of the amplitier A of the audion type. The grid 6 of this amplifier is supplied with negative potential by means of battery 7. The space current in' the output circuit of the amplifier is supplied by the battery 8 through the impedance 9. The filaments 10 and 11 of the oscillation generatorand amplifier are heated by the common battery 12. Amplified oscillations in the output circuit of the amplifier A are supplied by means of the transformer T to the input circuit of the harmonic generator G,which is a three-element vacuum tube of V the audion type.

The harmonic generator G is described and claimed in the'application of Kendall, for generator and generationof multiple frequencies, Serial No. 139,530, filed December 29, 1916, Patent No. 1,446,752, granted Fel ruary 27, 1923, and as noted therein the harmonic generator is overloaded as to produce a maximum distortion of they wave form supplied by the harmonic generator. Since a distorted Wave may be considered as composed of a pure sine wave of the same frequency as the distorted wave, and a plurality of harmonics having higher frequencies, it remains to selectthe particular higher frequency wnich it is desired to transmit.

The filament 13of the harmonic generator tea es the input circuit 21 of the modulator M.

s This modulator M is described and claimed in the patent to Hendrick J. Van der Bijl, 1,350,752, patented August 24, 1920. This modulator comprises a three-element vacuum tube 22 of the audion type, the grid 23 of which is maintained at a negative potential by means of the battery 24. The output circuit of tube 22 comprises condenser 25, which shunts the impedance 26 and the source This output circuit is tuned to the frequency to be transmitted by means of the condenser 28. The modulator M is coupled to antenna 29 by means of the transformer T By means of the line 30, amplified control oscillations of the frequency generated by the oscillator O are supplied to the filter F,

which selectively transmits oscillations of the control frequency to the transformer T coupled to the input circuit of the modulator M. The signal to be transmitted is transferred from the line L to the input circuit of the modulator M by'means of the transformer T The carrier oscillations supplied by the transformer are then modulated in accordance with both the control oscillations supplied by the transformer T and the signal currents in line L. The carrier oscillations, as modulated, are then transmitted from the antenna- 29 and received by the antenna 50, shown in Fig. 2. The antenna 50 is coupled by means of transformer T to the amplifier A theinput circuit of which tuned to the frequency of the oscillations to be received by means ofthe condenser 31. The amplifier A is similar to the amplifiers above described and is essentially unilaterall conducting for a purpose that will be later described. The output circuit of amplifier A is coupled to the detector '1), the input circuit of which is suitably tuned by means of the condenser 32. This detector is vacuum tube of the Well-known audion type. The output circuit of the detector D is coupled by means of the transformed T,,to the filterh .which selcc tively transmits detected control oscillations of the frequency generated by the oscillator 0. By means of the transformer T the output circuit of detector D is also coupled to the filter F which su 'ipresses all. frequenciesabove 2500 cycles and which transmits exclusively the detected audible signaling currents.

The filter F selectivelyztransmits the control oscillations to the power limitingtube and harmonic generator G which comprises a three-element vacuum tube of the audientype. The potential ofthe grit 33 f the de vice G is maintained positive by means of the battery 34. The positive grid attracts the electrons emitted by the filament 35, and I when the potential supplied to the deviceGg reaches a sulficiently high positive value, the

The device G operates as a harmonic gen erator by reason of'the fact that it is over-' loaded as noted above. Carrier frequency reinforcing 0SCllltt1QnS:fI0H1 the harmonic generator G are supplied to the amplifier A the input circuit of which is tuned by theva riable condenser 36. The phase of. the amplified reinforcing oscillations is properly adjusted by means of thc variable inductance 37, variable capacity 38 and variable resistance 89. Amplified reinforcing 'oscil l lations of the proper phase are'supplied to the resistance 40, which serves as asourceof potential. for the input circuit of theamplifier A the output circuitof: which is-coupled tothe input circuit 'of, detector D by.

means of the transformer T Oscillations from transformer T cannot,.howcver, be

transmitted through amplifier A to antenna 50, since amplifier; A is asymmetrically conducting and only permits the passage of currents in the direction-from the antennato the detector D. H

It will be apparent then thatcontrol oscillations supplied by the oscillator O and the signal currents sent over theline L will serve to modulate the carrieroscillations supplied by the transformer, T from thehar monic generator G. The modulated oscillations are transmitted from antenna 29 and are received by the antenna 50. Thedetector D supplies detected control oscillations'which are selectively transmitted by means of the filter F to the harmonic generator and power limiting tube G The device G operating as a harmonic generator, supplies reinforcing oscillations of the carrier frequency and these oscillations are amplified by the amplifier A The phaseof the, reinforcing]oscillations is properly adjustedby'means of the variable inductance 37, variable capacity 38 and variable resistance .39. Reinforcing.- oscillations having a proper phase with lGlittIOIltO the carrier oscillations are then amplified by the amplifier A and transferred to the input circuit of the detector D, where the reinforc ing oscillations: combine with the received modulated oscillations, thereby intensifying both the received control oscillations and the low frequency signal up to an amount which is limited by the capacity of the power limiting device G to pass control oscillations from filter F to amplifier A The object of this intensification is to reproduce in line L, the signal which originated in theline L. The signal currents are then supplied by means of the transformer T to the line L the filter F serving to suppress all the frequencies above the audible range.

Since the carrier oscillations and the reinforcing oscillations are derived from a common source, and since both are provided by changing the frequency of the oscillations supplied by that source, it follows that the carrier frequency and the reinforcing frequency will always be identical; which is esseutial for the proper reception of the telephonic signal. It is to be noted here that the selecting devices, i. e;, tuned circuits and filters. employed, willfproperly differentiate between the various frequencies involved,

even though the control frequency and its. multiple are subject to small variations.

The oscillation generators, amplifiers and harmonic generators shown n the remaining figures are similar to those described above.

Referring toFig. 3, there is shown a multiplex sending system in which the harmonic generator supplies a plurality of carrier frequencies, each of which may be modulated with a signal to be transmitted. The control oscillations supplied by the oscillation generator G are amplified by means of the am plifier A and amplified control oscillations are supplied by means of the transformer T to the input circuit of the harmonic generator G The several carrier oscillations supplied by the generator G are amplified 'b means of am )lifier A the outnut'circuit of which comprises the two tuned circuits 40 and 41, each of which is tuned to a different harmonic supplied by the harmonic generator G Carrier oscillations supplied by the tuned circuit 40 are amplified by means of the amplifier A from which amplified carrier oscillations are selectively transmitted by means of the filter F tothe modulator M Similarly, th tuned circuit-41 supplies amplified carrier oscillations of a frequency different from that supplied by thetuned circuit 40,130 the amplifier A which is coupled to the filter F which selectively transmits carrier oscillations to the modulator M The modulator M -comprises two threeclement vacuum tubes of the audion type, 42 and 43, which have their-input circuits connectedin opposition.v The signaling currentsaresupplied to the modulator M, by

means. of the line L pThe, transformer Tig translatingcircuits. The output circuit of the modulator M is coupled by means of the transformer T to the filter F which suppresses any low frecuiency signals that might otherwise be transmitted to the high frequency line L The modulator M is provided with the low frequency line L and this modulatoris coupled through the filter F to the high frequency line L By means of the coil 62 of the transformer T gsome of the control oscillations are supplied to the filter F which selectively transmits only oscillations of the control frequency to the line L in circuit with the high frequency line L The modulators M and M serve to suppress the unmodulated com ponent of the carrier wave and each trans mlts a pure modulated wave to the line T which is also supplied with control oscillations from the line/L V The oscillations transmitted to .the h gh frequency line L are received by the receiv The filter F ing' station shown in 4. connected to the line Ln, supplies control oscillations to the amplifier A coupled to [the harmonic generator which supplies reinforcing oscillations of the various ca"- rier frequencies to the amplifier A similar to the/amplifier A shown at the sending station in 3p In Fig; 4,. the tuned circuits :60 and 61 si1pplymnpl.ifiecl reinforcing oscillations of the carrier frequencies the amplifiers A and A. respectively. Amplified reinforcing oscillations are supplied by meansof the transformers TM and T to .the'input circuit of detector D1. The resistance 44, connected. across the line L serves as a source of modulated oscillations for the unila eral amplifier A which prevents the oscillations supplied by transformer T from being; transm ttedto the line L The amplifier A1 supplies amplified modulated oscilla tions to'the resistance 4-5, which serves as a source of potential for the detector 1);. The reinforcing oscillations" supplied by transformer T are combined with the received modulated osc llations to reproduce in the output circuit of the detector D the low frequency sio'nahwhich. original yleffected the modulation of the carrier oscillations at the sending; station shown in Fig. The output circuit the detector-D, is coupled to the filter F which suppresses all frequencies above the audible range.

The circuits associated with the output circuit of amplifier A are similar to those just described in connection with amplifier A 7 The filter F will separate out the control oscillations from the other transmissions and the tuned circuits will also isolate the proper multiples of the control frequency even though this frequency varies by small amounts. As both the carrier oscillations at the sending end and the reinforcing oscil- .lations are here derived as multiples of the same control oscillations, it is evident'that they will have identical frequencies.

1n the system shown in Figs. 1 and 2, the controloscillations were transmited as modulated oscillations of the carrier wave, while in Figs. 3 and 4, the control. oscillations are directly transmitted over the high'frequency line L;,.

The system shown in Fig. 5 is similar to that in Fig. 3, with the addition of means whereby carrier frequencies are provided which are not multiples of the control frequencies used and this serves to increase the number of carrier frequencies within a given range. The oscillation generator 0 shown in Fig. 5, will be considered as supplyingcontrol oscillations of a frequency equal to 3000 cycles which are amplified by means of amplifier A and transmitted by transformer T to the lIZlIIAOIfiC gGHQIQlLOI G Amplified carrier-frequencies are supplied. from the harmonic generator Ct. to

the amplifier A the output circuit of which contains the tuned circuits 46, 47 and 4-8; The circuits 47 and 48, by way of example, may be tuned to a frequency of 9000 cycles while the circuit 46 may be tuned to some other frequency, such as 12,000. It is, of course, obvious that any number of other tuned circuits may be provided in the output circuit of the amplifier A to provide sources of as many different carrier oscillations as desired. By means of the tuned circuit 47, carrier oscillations of frequency 9000 are supplied to the filter F which selectively transmits the carrier frequency 9000 to a modulator M similar to the modulators M and M already described. By means of the tunedcircuit l8, amplified oscillations of frequency 9000 are transferred to the input circuit of the combiner C.

It is shown in the application of Kendall, Serial No.-121,571, filed September 22, 1916, Patent No. 1,734,132, granted November 5, 1929, that if oscillations of two'diiferent frequencies are supplied to the input circuit of a device having a curved voltage-input current-output characteristic that there will appear in the output circuit frequencies equal to the sum and difierence of frequencies supplied to this device. As noted in the said application, No. 121,571, of Kendall, a vacuumtube of the audion type has a'curved characteristic, and the combiner C is such a I tube. r

The oscillator O maybe considered as generating Control oscillations of frequency 4000 cycles which areamplified-,byithe:am-'

plifier A and transmitted by the transformer T to theharmonlc gene1'ator:G

similar to the sending circuit associated with the oscillator 0 The harmonic generator G may supply carrier Oscillations of 'f'requenc 16,000 to the'amplifier A i- Amplified carrier oscillations of frequency 10,000 are supplied fromthe ainplifier'A through the filter F to the'modulato'r Ml. 'Amplified oscillations of frequency 16,000 are transferred by meanso'f tl e'trans'fon er T to the input circuit of the combiner'C, and amplified oscillations of frequencyQOOO are transferred by means of the transfori'ner Tfi" to the input circuit of this combiner; The output circuit of the combmer C then becomes a source of 'carrieroscillations of a fre-" quency 25,000, which are selectlvel'ytransmitted through the filter F g to the modulator M Low frequencies sent over the lines L L and L associated with'the' niodulators M M and-M respectively, effect a modulation of the carrier oscillations' -s up" plied to these modulators. The output circuits of the modulators M M andMg si lp ply modulated carrier oscillations"through the filters F F and, F 7 respectively to) the" high frequency line "L By means ofthe third winding on-the transformer T control oscillations generated 'by oscillator (hare transferred" to filter F o-Which 'selec tively transmits oscillations of 8000'cycles "to the line 'L Similarly, transformer T5 transmits control oscillations' 'gen'erated by oscillator 0 to filter F and the'nc'eto line L Each of these filters p'revents the'control oscillations transmitted to the line L from theother filters from interferingwith' the carrier oscillations supplied its'asso; ciated generator. Line L n is then adapted to transmit control oscillations'of frequency 8000 and 4000, and also. modulated oscillations having the carrier frequencies, 9000, 16,000 and 25,000. I V The number of control current generators shown in the drawings is only two,':it',1s obvious that any; number ofthese'may be used, and it is also obvious that other cornbiners may be used in order to produce vari one other frequencies which arenot multi les of the frequencies supplied to this combiner;

The line L in Fig; '5 is terminated inthe resistance 53, shown in Fig.6, which serves as a source of oscillations for the various cir cuits in shunt to this resistance." In shunt'to resistance 53 is the amplifier K the output circuit of which is connected'tothe filter which transmits control oscillations 'of affre quency of 3000 cycles.Controloscillations of this frequency are supplied from thefi'l ten-F to the harmonic generator G ,;c'oupled to the amplifier A the output circuit ofwhich contains the tuned'circuits 54, 55 and 56.-:- Theucircuit54-istunedto the same frequencyas the circuit46 in-Fig, 5. Y The circuit' 55uistu'ned to the frequency 9000 and' is coupled to thefilt'er'F which serves to se-f lectively transmit reinforcing oscillations ofthe carrier frequency 9000'to the;- input circuit of the detector D The circuit'56 is tuned to afrequency of 9000, and currentsof this'frequency are suppliedto' the input'circult of the combiner-C v Theamplifier A filter F and harmonic generatorGr are similar to'the corresponding elements A17; Fisk, Ge described above, the difference being inthe adjustment of these ele ments whereby filter F 20 selectively transmits control oscillations of a frequency of; 4000 cycles 't'o'the harmonic generator G which supplies reinforcing oscillations of frequency 1 0,000 to the amplifier A The'o'utput circuit of :amplifier A comprises the circuits 57 and 58 which are tuned to th'e reinforcingfrequency l6,000-, whic 'h is the 'same as 'cme of the carrier frequencies' Oscillations of frequency 9000, s'uppliedto thecombiner C by means'of'the tuned circuit 56, and oscil lations of frequency 16,000, supplied to this combiner by means of the tuned circuit 57, produce in the output circuit of this combineroscillations "of the reinforcing frequency 25,000, which are selectively transmitted through the filter F to; the detector D Also oscillations of'f'requen'c'y 16,000 are supplied from the tuned circuit 58 through filter F5 to the inputcircuit of'th'e'detectorDg. Amplifier A 1 isconnected inshunt to the resistance 53, and amplified modulated oscil lations'fare s'upplie'dby thisainplifier to'de tector D The amplifier A is essentially milaterally' conducting in order that" the reto'lthe high-frequency lin e Lg are similarto the circuits associated'with' D; which have been describedfi 5 lowffreq'u encysignaling lines at the transmittingand-receiving stations are shown supplied with telephone transmitting "and receiving apparatus; Obviously, this system is"riotf1i 1 nitedtotelephonic signaling, 1 but maybe equa-lly'well. applied to telegrapli'or other signaling or,,in fact, to mission of any kind.

Whatis claimed is: p 1. The method comprising modulating a selective trans- -carrier wave in accordance with control energy,: transmitting and detecting said wave to reproduce said control energy at a receiving station, and utilizing said detected control energy to reinforce ,said transmitted carrier Wave.

2. The method of transmission comprising modulatinga carrier wave in accordance with 7 control energy, transmitting said modulated wave and detecting it at a receiving station to reproduce said control energy, amplifying said control energy and utilizing said amplified control energy to reinforce said transmitted carrier wave.

3..The method comprising modulating a carrier wave in accordance with control energy, transmitting and detecting said wave 1 to reproduce said control energy at a receiving station, and deriving from said detected control energy a'reinforcing wave of the frequency of said carrier wave.

4. The method which comprises generating a wave of low frequency, obtaining a wave of a'single higher frequency from said wave of low frequency, freeing the wave so obtained from waves of other than said single frequency and modifying said wave of higher frequency in accordance with said wave ofprises modulating a carrier wave in accordance with both signals and control energy, transmitting said modulated wave, receiving and detecting saidwave, producing reinforcing oscillations from the control energy component of said wave, and combining said reinforcing oscillations with said received wave. v

7. The method of carrier wave transmission which comprises modulating a carrier wave .in accordance with-control oscillations, transmittingsaid modulated wave to a distant station and there detecting it to obtain control oscillations, producing locally generated oscillations at said receiving station for combination with saidreceived carrier oscillations, and causing said control oscillations to control the frequency difi'erenceof said carrier and said locally generated oscillations. I r

8. The method of telephony comprising simultaneously transmitting speech current energy and control energy as modulations of the same carrier wave, receivingsaid transmitted energy, causing said received control energy to produce reinforcing oscillations, and utilizing said reinforcing oscillations to reinforce said speech current energ a 9. The combination with means or generating a wave of low frequency, of means for obtaining therefrom a wave of higher frequency, means for freeing the higher frequency wave so obtained from waves of other than its single frequency and means for modifying'said wave of higher frequency in accordance with said wave of'low frequency.

10. The method of homodyne transmission which comprises modulating a carrier wave in accordance with controlenergy, transmitting said wave and detecting it at a receiving station to reproduce said control energy, deriving from said reproduced control energy reinforcing oscillations of the frequency of said carrier wave, and regulating the phase of said reinforcing oscillations with respect to that of said transmitted carrier wave.

. 11. The method of reinforcing a received modulated carrier wave which comprises detecting said wave, deriving from a detected component thereof reinforcing oscillations of the frequency of said carrier wave, and combining said oscillations with said received wave.

, 12. The method of reinforcing a received modulated carrier wave which comprises detecting said wave, deriving from a detected component thereof reinforcing oscillations of the frequency of said wave, and combining said oscillations with said received wave after limiting the magnitude of said oscillations in order to prevent generation of con tinuous oscillations,

13. 'A. multiplex transmission system comprising means for transmitting control oscillation energy, means-for receiving said transmitted control oscillation energy, means for deriving from said received energy harmonic frequency oscillations, and means for amplifying said harmonic frequency oscillations to substantially the same magnitude.

14. A transmission system comprising means for transmitting simultaneously, modified carrier oscillations and control energy, means for receiving said oscillations and said energy, means for causing said received control energy to produce reinforcing oscillations ofsaid carrier frequency, and means for adjusting the phase of said reinforcing oscillationswith respect to that of said received carrier oscillations. V

v 15. A' transmission system comprising means for producing a carrier wave and modulating said wave in accordance with signals, meansv for transmitting and means for receiving said modulated wave, means for detecting said received carrier wave to reproduce said signals, means including a fre .quency selective circuit for supplying reinforcing oscillations of the carrier frequency to said detecting means, and additional means for adjusting the phase of said reinforcing oscillations. V V

16. A transmission system comprising means for producing a carrier Wave and mod ulating saidwavc in accordance with both signals and control energy, means for transmitting and means for receiving said modulated wave, means for detecting said received carrier wave to reproduce said signals and said control energy, means for deriving from said detected control energy reinforcing oscillations of said carrier frequency, and means for adjusting the phase of said reinforcing oscillations with respect to that of said received carrier wave.

17. A signaling system comprising a detector, means for supplying a modulated wave of a given carrier frequency thereto, means for deriving reinforcing oscillations of said carrier frequency from said detected wave and for impressing said oscillationsupon said detector simultaneously with said wave, and means for preventing continuous transfer of energy from said detector back to itself.

18. A signaling system comprising a detector, means supplying to said detector car-- rier oscillations modulated in accordance with control energy, means for detecting said con trol energy and derivingtherefrom reinfor ing oscillations of said carrier frequency, and means 'for applying said reinforcing oscillations to said detector, said several means comprising a power-limiting device to prevent continuous transfer of energy from said detector back to itself.

19. The method of signaling comprising transmitting signals and control waves as components of a complex wave, utilizing at a receiving point the control wave component to control a locally derived wave and reproducing the signal by cooperation of the locally derived wave with the signal component of the complex wave. I c r 20. In a system of signaling in which wave energy is produced and is added at a receiving point to the wave received from thetrausmitting point, the method of signal reception comprising utilizing a component of the re ceived wave to control the frequency of the wave so added at a receiving point.

21. The method of synchronizing the operation of apparatus at a certain relatively low frequency when the apparatus is located at two places far apart, which consists ingenerating at one place an electric current of a higher frequency definitely related to said first mentioned frequency, and transmitting such higher frequency current to the other place and applying it to determine the operation of the apparatus there at said first mentioned frequency.

22. The method of synchronizing apparatus'to a certain relatively low frequency at two places, which consists in transmitting a current of harmonic frequency from the app'aratus at one place and applying it to effect synchronization at thesaid relatively low frequency at the other place.

The method of synchronizing apparatus to a certain relatively 'low frequency at two places, which consists in generating at one placea current of higher frequency and definitely relatedin phase and frequency to the said current of relatively low frequency and transmitting said higher frequency-current and applying ita't the other place to effect synchronization at the said relatively low frequency.

24. In combination, atransmission medium incapable oftransmitting currents of relatively low frequency, apparatus at two places to be synchronized ata relatively low frequency, and means for generating a harmonic current at one of those places andy transmitting it over said medium and applying it at the other place to synchronize the apparatus there'at with the apparatus where the said harmonic current was generated.

25. In a transmission system, a band filter for transmitting currents of only a limited range of frequencies, a circuit for supplying current to said band filter, a source of current of single frequency outside the range of said filter connected to said circuit, and modulating means in saidcircuit whereb-ythefie quency of the current from said source is changed to a frequency wlthin :the range of said band filter.

26. In a signaling system, means for generating fa sustained alternating current, means for continuously modulating said alternating current according to message signals, means for receiving said modulated current, means for dividing said received current into message signalcurrents and into,

synchronizing signal currents, and means synchronized by said synchronizing signal currents. V c

27. In a signaling system, means for generating a sustained alternating current, means for continuously modulatingsaid alternating current according to message signals, means for receiving said modulated current to produce message signals, and means synchronized byutilizing a portion of said received current.

28. In a signaling system, means for generating a sustained alternating current, means for continuously modulating said al-' ternating current according to message signals,means forreceiving said modulated curutilizingrsaid received current both for reproducing the signals and for synchronizrent, means to be synchronized and means for means for continuously modulating said alternating current according to message signals, means for. receiving said modulated current to produce message signals, synchronizing means, and means for utilizing energ derived from said received current for controlling said synchronizing means.

30.=The method of receiving a signal modiiiedhigh frequency wave Which comprises utilizing a portion of the energyoit' said Wave to synchronize a'local device for producing Waves and combining other energy or said wave with Wave energy from said source.

Elle-The method of signaling which consists'in transmitting a comgosite Wave-train including a signal-modulated component and a synchronizing component, deriving from the synchronizing component at the receiving station a local Wave-train separate from but synchronous with the received signal-modulated component, amplifying said synchronous looul ave-train separately from the received signal-inodnlated component, and combiningr'the amplified synchronous local wave-train with the received signal-modulated component to produce signal 'conveying eifect. V r f 32. A signaling system comprising means for generating a' carrier current,'means for modulating said current at the same ime both in accordanec' with signals and control impulses, means for receiving said modulated current, means for detecting, means to be synchronized, and means for utilizing the detected control impulses for synchronizing.

In Witness whereof, I hereunto subscribe my name this 19th day of October, 1928.

' BURTON lV. KENDALL. 

